The Himalayan mountains were formed by the collision of the Indian and Eurasian tectonic plates. This is an example of continental-continental convergence, where two continental plates collide with each other.

Here's a breakdown of how it happened:

* The Indian Plate was moving northward, propelled by the Indian Ocean Ridge.

* The Eurasian Plate was relatively stationary.

* The collision of these two massive landmasses caused the denser Indian Plate to subduct beneath the Eurasian Plate. However, both plates were too buoyant to be completely subducted.

* The immense pressure from the collision caused the rocks of both plates to fold, buckle, and uplift, creating the Himalayan Mountains.

This process is still ongoing, which is why the Himalayas are still growing today. This growth, although slow, is responsible for frequent earthquakes and volcanic activity in the region.